Hypo-osmotic shock and the role of freshwater input on the post-bloom recovery of Cyanothece sp. in Lake St Lucia, South Africa

ABSTRACT

Modern anthropogenic modifications to aquatic environments, specifically hydrodynamic alterations, play a major role in cyanobacterial bloom potential. In shallow-water ecosystems, salinity is a driver of microbial communities and increases in salinity may facilitate the bloom potential of competitive cyanobacteria. This study investigated the osmotic response and mortality rate of a persistent bloom-forming (18 months) cyanobacterium, Cyanothece sp., isolated from Lake St Lucia (South Africa), after sudden hypo-osmotic shock. Laboratory experiments were performed with Cyanothece sp. cells to observe osmotic responses and to determine mortalities during salinity reductions. In general, Cyanothece sp. cells conformed to the external medium at all salinity levels used in the treatments, reducing their intracellular osmolality in response to salinity downshifts. There were limits to this, however, as successful downshift was not observed during the freshwater exposure treatment. Highest mortality rates occurred in cultures that were grown at high salinities (i.e. 180, 240 and 300 units), as well as in treatments with large salinity downshifts (e.g. 120 salinity unit reduction and direct freshwater exposure). Cyanothece sp. has been shown to sustain bloom status for long periods. Its ability to tolerate high salinities and respond to sudden salinity downshifts is an important factor in its bloom success. However, it is still susceptible to reductions in salinity that may occur due to heavy rainfall and floods, which highlights the importance that freshwater plays in the bloom ecology of a persistent bloom-forming halotolerant cyanobacterium, as freshwater inputs markedly influence the physiology of the cells and their survival ability.

SUBJECT EDITOR:

• Audrey J. Geffen

KEYWORDS:

• Cyanothece sp
• Lake St Lucia
• bloom persistence
• osmoregulation
• hypo-osmotic shock
• osmolality

Acknowledgements

This work is based on research supported by the South African Research Chairs Initiative of the Department of Science and Technology (DST) and the National Research Foundation (NRF) of South Africa. Any opinion, finding and conclusion or recommendation expressed in this material is that of the author(s) and the NRF does not accept any liability in this regard.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by the National Research Foundation (NRF, Pretoria, Unique Grant No. 84375), and the South Africa–Netherlands Research Programme on Alternatives in Development (SANPAD, Durban, Project 10/90).